In microdisplay-based display systems used to display images by projecting the images onto a display plane, the contrast ratio of the display system has a significant impact on the perceived quality of the system. A display system's contrast ratio can be defined as a ratio of the brightest displayable gray scale (typically, pure white) to the darkest displayable gray scale (usually, pure black). The display of pure black can often be difficult to achieve in many display systems, since it is typically not feasible to simply turn off a light source used to display the images when there is a need to display the pure black. Rather, light from the light source is normally redirected so that the amount of light projected onto the display plane is minimized. Since it is not possible to turn off the light, it can be very difficult to achieve pure black.
In a display system that makes use of positional micromirror light modulators, the array of micromirror light modulators is commonly referred to as a DMD (digital micromirror device), wherein micromirrors are mounted on hinges and are arranged in an array. The micromirrors traverse along an axis to either reflect light from a light source onto the display plane or away from the display plane, dependent upon image data being displayed. When a micromirror is in a position to reflect light away from the display plane, there is still a possibility of scattered light from the micromirror and its underlying support structure (hinges, hinge supports, landing pads, electrical conductors, and so forth) to reach the display plane. The scattered light that reaches the display plane can lighten the darkest displayable gray scale and therefore reduce the contrast ratio of the display system.
The underlying support structure of the micromirror can be coated with an antireflective coating to help reduce the light scattering. However, since the micromirror must be able to reflect as much light as possible (to maximize the brightness of the display system), to coat the micromirror with an antireflective coating will only reduce the perceived quality of the images displayed by the display system.
With reference now to FIGS. 1a through 1d, there are shown diagrams illustrating an isometric view and a side, cross-sectional view of a typical micromirror 100 and several techniques to help reduce light scatter from a via present in the surface of the micromirror 100. The micromirror 100 is a planar mirror that has a four-sided shape, however, the shape and size of the micromirror 100 can be dependent upon the arrangement of the array of micromirrors in the DMD, as well as the desired density of the DMD, the fabrication process technology, and so forth. The micromirror 100 includes a via 105 that is part of the physical structure of the micromirror 100 and can be used to attach the micromirror 100 to a hinge (not shown).
Light (shown as light beam 110) from a light source (not shown) can strike the surface of the micromirror 100 and reflect (shown as light beam 111) with an angle of reflection that is equal to an angle of incidence. However, light that strikes the via 105 will tend to scatter (shown as small light beams 112 and 113). Although the surface area of the via 105 may be small compared to the surface area of the micromirror 100, the amount of light scattered by the via 105 can contribute to a decrease in the contrast ratio of the display system.
It is possible to reduce the amount of light scattered by the via 105 by reducing the surface area of the via 105, as shown in FIG. 1c. With a smaller surface area, a ratio of micromirror surface area to via surface area decreases, therefore, the percentage of scattered light to non-scattered light is reduced. Another possible solution that can be used to reduce the amount of light scattered by the via 105 is to place an antireflective coating 120 inside the via 105 so that light that enters the via 105 will tend to be absorbed by the antireflective coating 120 and not scatter, as shown in FIG. 1d. Another possible solution involves the deposition of a coating onto the mirror surface and the mirror surface can then be polished to remove the coating and leaving the coating over the via 105.
One disadvantage of the prior art is that reducing the surface area of the via 105 can result in making the formation of the physical structure of the micromirror more difficult. Furthermore, a reduction in the size of the via 105 may reduce the reliability of the attachment of the micromirror 100 to the hinge (reduced step coverage).
Another disadvantage of the prior art is that the placement of an antireflective coating 120 into the via 105 can be hard to control and some of the antireflective coating can end up deposited onto the surface of the micromirror 100 or the via 105 may not be fully coated, resulting in either a loss of brightness or scattered light.
Yet another disadvantage of the prior art is that the polishing of the mirror surface to remove the coating can remove some of the material of the mirror surface, thereby potentially damaging the mirror surface. Additionally, the removal of the material of the mirror surface can weaken the mirror surface, making the mirror open to damage during use.